The present invention relates to a method for fabricating semiconductor device, more specifically a method for fabricating a semiconductor device including an optical integrated circuit, etc. for use in optical communication, photocoupling, etc.
In an optical semiconductor device of an InP-based material, horizontal light confinement depends on refractive index differences between the InGaAsP core and the InP buried layer, which guide light. As a structure for realizing such light confinement, the buried hetero (BH) structure used in semiconductor lasers is known.
A method for fabricating a semiconductor laser of the BH structure will be explained with reference to FIGS. 10A-10E. FIGS. 10A-10E are sectional views of the semiconductor laser in the steps of the method for fabricating the semiconductor laser.
As shown in FIG. 10A, an InGaAsP/InGaAsP multi-quantum well layer 102, a p type InP clad layer 104 are formed sequentially on an n type InP substrate 100.
Then, as shown in FIG. 10B, an SiO2 film 106 as an etching protection film is formed on the p type InP clad layer 104, and an active layer mesa stripe 108 are formed by dry etching. The active layer mesa stripe 108 has  less than 011 greater than  direction.
Subsequently, as shown in FIG. 10C, with the SiO2 film 106 as a selective growth mask, a p type InP buried layer 110 and an n type InP buried layer 112 are sequentially crystal-grown on the InP substrate 100 around the active layer mesa stripe 108 by metal organic vapor phase epitaxy (MOVPE). In the crystal growth by the MOVPE, a chlorine-based gas, such as CH3Cl or others, is added so that, as shown in FIG. 10C, the crystal growth of the n type InP buried layer 112 stops at a (111) B plane as the growth stop face. Thus, the n type InP buried layer 112 can be formed, not growing over the SiO2 film 106.
Following the formation of the n type InP buried layer 112, the SiO2 film 106 is removed by etching using HF, and a p type InP clad layer 114 and a p type InGaAs contact layer 116 are sequentially formed on the entire surface.
Finally, an n type electrode 118 is formed on the underside of the n type InP substrate 100, and a p type electrode 120 is formed on a p type InGaAs contact layer 116. Thus, the fabrication of the semiconductor laser of the BH structure is completed.
Recently in the optical communication technique, for multi-wavelength communication and high-speed light modulation, optical integrated circuits having photodividers, photocouplers, photomodulators, photoswitches, etc. integrated have become key devices. Such optical integrated circuits are fabricated by the same method as the semiconductor laser of the BH structure described above.
However, in fabricating the optical integrated circuit of the BH structure, the step of forming the buried layer after forming the active layer mesa stripe has problems although the step has no problem in fabricating the semiconductor laser.
The resonator of the above-described semiconductor laser has  less than 011 greater than  direction. As shown in FIG. 10C, the crystal growth of the n type InP buried layer 112 stops at the (111) B plane as the growth stop plane, so that the n type InP buried layer 112 can be formed, not growing over the selective growth mask.
On the other hand, the wave guide of the optical integrated circuit has the function of coupling various devices and, for the function, has parts of different directions, a branch part and a terminal part. FIG. 11 is a view of a structure of the active layer mesa stripe of the optical integrated circuit with SiO2 film as the selective growth mask. As shown in FIG. 11, a branch part 122 and a terminal part 124 are formed in the active layer mesa stripe 121 of the optical integrated circuit, and a part 126 of a direction other than  less than 011 greater than  direction is formed in the active layer mesa stripe. In order to form the buried layer in such active layer mesa stripe 121 the SiO2 film 128 is formed as a selective growth mask.
In a case that the selective growth mask is formed at the branch part 122 or the terminal part 124, or a part 126 of a direction other than  less than 011 greater than  direction, no specific growth stop plane is present in forming the buried layer. Accordingly, the buried layer grows over the selective growth mask. FIGS. 12 A, 12B and 12C respectively show growth of the buried layer at the branch part 122, the terminal part 124 and the part 126 of a direction other than  less than 011 greater than  direction. As shown in FIGS. 12A-12C, at the branch part 122 and the others the buried layer 120 is formed, growing over the SiO2 film 128, which is the selective growth mask formed on the active layer mesa stripe 121. Such over growth of the buried layer 130 cannot be prevented even by addition of chlorine gas at the time of the crystal growth.
In the optical integrated circuit, stacking dislocations unpreferably occur in a part where the above-described over growth has taken place. Furthermore, the overhang of the buried layer formed by the over growth form voids therebelow in the step of removing the selective growth mask to form a clad layer and a contact layer. FIG. 13 shows one example of the sectional configuration of the optical integrated circuit having a void formed by the over growth of the buried layer.
As shown in FIG. 13, a p type InP buried layer 134 and an n type InP buried layer 136 are formed in layers on both sides of the active layer mesa stripe 121 formed on an n type InP substrate 132. Furthermore, on the upper surface of these layers a p type InP clad layer 138, a p type InGaAs contact layer 140 are sequentially formed. The n type InP buried layer 136 has overhanging parts 142 growing over the active layer mesa stripe 121. Below the overhanging parts 142 the p type InP clad layer 138 is not formed, forming voids 144.
The stacking dislocation and void due to the above-described over growth of the buried layer cause deflection of a refractive index which hinders waveguide of light, and furthermore cause electric characteristic deterioration in the operation of the device.
An object of the present invention is to provide a method for fabricating semiconductor device which can grow a buried layer on a projected structure, such as a mesa stripe, without over growth.
The above-described object is achieved by a method for fabricating semiconductor device comprising: the step of forming a buried layer of III-V group semiconductor with Se or S added in an above 5xc3x971018 cmxe2x88x923 concentration against a projected structure including a surface with a mask formed on, at a peripheral part of the mesa structure without the mask formed on.
In the above-described method for fabricating semiconductor device it is possible that the III-V group semiconductor is InP.
In the above-described method for fabricating semiconductor device it is possible that the projected structure is a mesa stripe having a branching part.
In the above-described method for fabricating semiconductor device it is possible that the projected structure is a mesa stripe having a terminal part.
In the above-described method for fabricating semiconductor device it is possible that the projected structure is a mesa stripe having a  less than 011 greater than  direction part and a part of a direction other than  less than 011 greater than  direction.
In the above-described method for fabricating semiconductor device it is possible that a gas containing chlorine is introduced when the buried layer is formed.
In the above-described method for fabricating semiconductor device it is possible that the mask is a film of silicon oxide and/or silicon nitride.